Driving circuit for light-emitting element with burst dimming control

ABSTRACT

A detection resistor is arranged on a path of an LED string. A controller generates a gate pulse signal having a duty ratio adjusted such that the voltage drop at the detection resistor matches a predetermined reference voltage. Furthermore, the controller receives a dimming control signal which indicates the duty ratio of the burst dimming operation, and generates a corresponding burst dimming pulse. According to the gate pulse signal, the first driver drives an output circuit configured to generate a driving voltage. A second driver switches a path of a driving current between connection and disconnection according to the burst dimming pulse. A duty ratio detection unit controls the frequency of the gate pulse signal according to the duty ratio indicated by the dimming control signal.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 13/992,069, filed on Jun. 6, 2013, the entirecontents of which are incorporated herein by reference. The 13/992,069application is the U.S. National Stage of Application No.PCT/JP2011/006739, filed on 1 Dec. 2011. Priority under 35 U.S.C.§119(a) and 35 U.S.C. §365(b) is claimed from Japanese Application No.2010-273461, filed 8 Dec. 2010, the disclosure of which is alsoincorporated herein by reference.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to a driving technique for alight-emitting element, and particularly to a dimming technique for sucha light-emitting element.

Description of the Related Art

In recent years, as a backlight of a liquid crystal panel or as anillumination device, a light-emitting apparatus is employed, which isconfigured using a light-emitting element such as an LED (light-emittingdiode) or the like. FIG. 1 is a circuit diagram which shows an exampleconfiguration of a light-emitting apparatus according to a comparisontechnique. A light-emitting apparatus 1003 includes an LED string 6, anda switching power supply 1004.

The LED string 6 includes multiple LEDs connected in series. Theswitching power supply 1004 is configured to step up an input voltageVin input to an input terminal P1 so as to output a driving voltage Voutto one end of the LED string 6 connected to an output terminal P2.

The switching power supply 1004 includes an output circuit 102 and acontrol IC 1100. The output circuit 102 includes an inductor L1, aswitching transistor M1, a rectifier diode D1, and an output capacitorC1. The control IC 1100 adjusts the driving voltage Vout by controllingthe on/off duty ratio of the switching transistor M1.

A PWM dimming switch (transistor) M2 and a current detection resistor R1for current detection are arranged on a path of the LED string 6. Acontroller 1010 generates a pulse-width modulated (PWM) burst dimmingpulse G2 having a duty ratio that is controlled according to an externaldimming control signal PWMDIM such that it matches the target luminancelevel. A driver DR2 performs switching of the PWM dimming switch M2according to the burst dimming pulse G2.

A voltage drop (detection voltage) V_(R1), which is proportional to thedriving current I_(DRV) that flows through the LED string 6, occurs atthe detection resistor R1. An error amplifier EA1 amplifies thedifference between the detection voltage V_(R1) and a reference voltageV_(REF) so as to generate a feedback voltage V_(F3). The controller 1010generates a gate pulse signal G1 that is subjected to pulse modulationaccording to the feedback voltage V_(FB). A driver DR1 performsswitching of the switching transistor M1 according to the gate pulsesignal G1.

With such a configuration, feedback control is performed such that therelation I_(DRV)=V_(REF)/R1 holds true, thereby allowing the LED string6 to emit light with a luminance level that corresponds to the referencevoltage V_(REF) (current dimming).

Furthermore, the light-emitting time of the LED string 6 is controlledaccording to the on/off duty ratio of the PWM dimming switch M2, therebyadjusting the effective luminance (PWM dimming or burst dimming).

RELATED ART DOCUMENTS Patent Documents

[Patent document 1]

Japanese Patent Application Laid Open No. 2009-261158

The contrast ratio of a liquid crystal panel can be improved bydynamically controlling the luminance of the LED string 6. That is tosay, by reducing the minimum luminance of the LED string 6, the contrastratio of the liquid crystal panel can be improved.

SUMMARY OF THE INVENTION

The present invention has been made in view of such a situation.Accordingly, it is an exemplary purpose of an embodiment of the presentinvention to provide a driving circuit which is capable of driving anLED string at a luminance level that is as reduced as possible.

An embodiment of the present invention relates to a driving circuitconfigured to supply a driving voltage and a driving current to alight-emitting element. The driving circuit comprises: a detectionresistor arranged on a path of the light-emitting element; a controllerconfigured to generate a gate pulse signal having a duty ratio that isadjusted such that voltage drop across the detection resistor matches apredetermined reference voltage, to receive a dimming control signalwhich indicates the duty ratio of the burst dimming, and to generate aburst dimming pulse according to the dimming control signal; a firstdriver configured to drive, according to the gate pulse signal, aswitching power supply configured to generate the driving voltage; asecond driver configured to switch the state according to the burstdimming pulse between a state in which a path of the driving current isconnected and a state in which the path of the driving current isdisconnected; and a duty ratio detection unit configured to control thefrequency of the gate pulse signal according to the duty ratio indicatedby the dimming control signal.

Another embodiment of the present invention also relates to a drivingcircuit. The driving circuit comprises: a current source arranged on apath of the light-emitting element; a controller configured to generatea gate pulse signal having a duty ratio that is adjusted such thatvoltage drop across the current source matches a predetermined referencevoltage, to receive a dimming control signal which indicates the dutyratio of a burst dimming, and to generate a burst dimming pulseaccording to the dimming control signal; a first driver configured todrive, according to the gate pulse signal, a switching power supplyconfigured to generate the driving voltage; a second driver configuredto switch the state according to the burst dimming pulse between a statein which a path of the driving current is connected and a state in whichthe path of the driving current is disconnected; and a duty ratiodetection unit configured to control the frequency of the gate pulsesignal according to the duty ratio indicated by the dimming controlsignal.

With such embodiments, by changing the switching frequency of theswitching transistor of the switching power supply according to the dutyratio of the burst dimming operation, such an arrangement is capable ofcontrolling the luminance of the light-emitting element according to theduty ratio even in a range in which the duty ratio is small.

Also, the duty ratio detection unit may be configured to raise thefrequency of the gate pulse signal as the duty ratio indicated by thedimming control signal becomes lower.

Yet another embodiment of the present invention relates to alight-emitting apparatus. The light-emitting apparatus comprises: alight-emitting element; and a driving circuit according to any one ofthe aforementioned embodiments, configured to drive the light-emittingelement.

Yet another embodiment of the present invention relates to an electronicdevice. The electronic device comprises: a liquid crystal panel; and theaforementioned light-emitting apparatus arranged as a backlight of theliquid crystal panel.

It should be noted that any combination of the aforementioned componentsmay be made, and any component of the present invention or anymanifestation thereof may be mutually substituted between a method,apparatus, system, and so forth, which are effective as an embodiment ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example, only, withreference to the accompanying drawings which are meant to be exemplary,not limiting, and wherein like elements are numbered alike in severalFigures, in which:

FIG. 1 is a circuit diagram which shows an example configuration of alight-emitting apparatus according to a comparison technique;

FIG. 2A is a diagram which shows the relation between the duty ratio ofa burst dimming pulse and a driving current in the light-emittingapparatus shown in FIG. 1, and FIG. 2B is a waveform diagram which showsthe burst dimming pulse and a gate pulse signal in the light-emittingapparatus shown in FIG. 1;

FIG. 3 is a circuit diagram which shows a configuration of alight-emitting apparatus according to a first embodiment;

FIG. 4A is a circuit diagram which shows a first example configurationof a duty ratio detection unit shown in FIG. 3, and FIG. 4B is awaveform diagram which shows the operation of the duty ratio detectionunit shown in FIG. 4A;

FIG. 5 is a circuit diagram which shows a second example configurationof the duty ratio detection unit shown in FIG. 3;

FIGS. 6A and 6B are waveform diagrams each showing the operation of thelight-emitting apparatus shown in FIG. 3, and FIG. 6C is a diagram whichshows the relation between the duty ratio of the burst dimming pulse andthe driving current in the light-emitting apparatus shown in FIG. 3; and

FIG. 7 is a circuit diagram which shows a configuration of a drivingcircuit according to a second embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Description will be made below regarding preferred embodiments accordingto the present invention with reference to the drawings. The same orsimilar components, members, and processes are denoted by the samereference numerals, and redundant description thereof will be omitted asappropriate. The embodiments have been described for exemplary purposesonly, and are by no means intended to restrict the present invention.Also, it is not necessarily essential for the present invention that allthe features or a combination thereof be provided as described in theembodiments.

In the present specification, the state represented by the phrase “themember A is connected to the member B” includes a state in which themember A is indirectly connected to the member B via another member thatdoes not affect the electric connection therebetween, in addition to astate in which the member A is physically and directly connected to themember B.

Similarly, the state represented by the phrase “the member C is providedbetween the member A and the member B” includes a state in which themember A is indirectly connected to the member C, or the member B isindirectly connected to the member C via another member that does notaffect the electric connection therebetween, in addition to a state inwhich the member A is directly connected to the member C, or the memberB is directly connected to the member C.

The present inventors have investigated techniques for reducing theminimum luminance of the light-emitting apparatus 1003 shown in FIG. 1.There are two approaches for reducing the minimum luminance. In thefirst approach, the driving current I_(DRV) is reduced by means ofcurrent dimming. In the second approach, the duty ratio of the burstdimming is reduced.

With the first approach, the driving current I_(DRV) can be reduced byreducing the reference voltage V_(REF). However, the input voltage rangeof the error amplifier EA1 configured to receive the reference voltageV_(REF) and the detection voltage V_(R1) has a lower limit. Thus, thisapproach has a limitation.

In order to solve such a problem, the second approach has beeninvestigated. FIG. 2A is a diagram which shows the relation between theduty ratio of the burst dimming pulse G2 and the driving current I_(DRV)in the light-emitting apparatus 1003 shown in FIG. 1. Over a duty ratiorange of the burst dimming pulse G2 that is greater than a certain lowerlimit β (0.04%), the driving current I_(DRV) changes according to theduty ratio. However, when the duty ratio becomes smaller than the lowerlimit β (0.04%), the driving current I_(DRV) sharply drops. That is tosay, by reducing the lower limit β, such an arrangement is capable offurther reducing the minimum luminance by means of the second approach.

The present inventors have investigated the reasons why there is a lowerlimit β on the duty ratio, and a method for reducing the lower limit β.It should be noted that the following investigation has been uniquelymade by the present inventors, and is by no means within the scope ofcommon and general knowledge of those skilled in this art.

In the burst dimming operation, in the off period of the PWM dimmingswitch M2, the feedback signal is disconnected, which stops theswitching of the switching transistor M1. In the on period T_(ON) of thePWM dimming switch M2, the feedback operation is effectively performedaccording to the detection voltage V_(R1). In this period, the dutyratio of the switching transistor M1 is adjusted, thereby stabilizingthe output voltage Vout. Accordingly, as the pulse width (duty ratio) ofthe burst dimming pulse G2 is reduced, i.e., as the on period isreduced, the period in which the feedback operation is effectivelyperformed becomes shorter. FIG. 2B is a waveform diagram which shows theburst dimming pulse G2 and the gate pulse signal G1 in thelight-emitting apparatus 1003 shown in FIG. 1.

When the pulse width (on period T_(ON)) of the burst dimming pulse G2approaches the period (switching period T_(SW)) of the gate pulse signalG1, the number of pulses of the gate pulse signal G1 included in the onperiod T_(ON) of the burst dimming pulse G2 fluctuates depending on thephase of the gate pulse signal G1. That is to say, at a certain phaseG1(1) of the gate pulse signal G1, there are two pulses of the gatepulse signal G1 in the on period T_(ON). On the other hand, at anotherphase G1(1) of the gate pulse signal G1, there is only a single pulse inthe on period T_(ON). Such fluctuation leads to a reduction in theenergy stored in the inductor L1, resulting in a problem in that theoutput voltage Vout cannot be maintained at its target value. Thepresent inventors have reached such an idea which explains thecharacteristics shown in FIG. 2A.

[First Embodiment]

FIG. 3 is a circuit diagram which shows a configuration of alight-emitting apparatus 3 according to a first embodiment. Anelectronic device 2 is configured as a battery-driven device such as alaptop PC, a digital still camera, a digital video camera, a cellularphone terminal, a PDA (Personal Digital Assistant), or the like, andincludes a light-emitting apparatus 3 and an LCD panel (Liquid CrystalPanel) 5. The light-emitting apparatus 3 is provided as a backlight ofthe LCD panel 5.

The light-emitting apparatus 3 includes an LED string 6 configured as alight-emitting element and a driving circuit 4 for the LED string 6.

The LED string 6 includes multiple LEDs connected in series. The drivingcircuit 4 includes a step-up DC/DC converter, and is configured to stepup the input voltage (e.g., battery voltage) Vin input via an inputterminal P1 so as to output an output voltage (driving voltage) Vout viaan output terminal P2. One end (anode) of the LED string 6 is connectedto the output terminal P2.

The driving circuit 4 includes a control IC 100 and an output circuit102. The output circuit 102 includes an inductor L1, a rectifier diodeD1, a switching transistor M1, and an output capacitor Cl. The outputcircuit 102 has a typical topology, and accordingly, description thereofwill be omitted.

A switching terminal P4 of the control IC 100 is connected to the gateof the switching transistor M1. The control IC 100 adjusts the on/offduty ratio of the switching transistor M1 by means of feedback controlso as to provide the output voltage Vout required to allow the LEDstring LED 6 to emit light, and so as to maintain the light emission ofthe LED string 6 at a target luminance. It should be noted that theswitching transistor M1 may be configured as a built-in component of thecontrol IC 100.

A PWM dimming switch M2 is arranged on a path of the LED string 6, andspecifically between the cathode of the LED string 6 and the groundterminal. By adjusting the on/off ratio (duty ratio) of the PWM dimmingswitch M2, such an arrangement is capable of adjusting the ratio betweenthe on time and the off time of the LED string 6, thereby adjusting theeffective luminance level of the LED string 6. Such an operation will bereferred to as “burst dimming (PWM dimming)”. A dimming control signalPWMDIM is input to a PWMDIM terminal. The dimming control signal PWMDIMis a signal which indicates the duty ratio of the burst dimming, and isconfigured as a pulse width modulated pulse signal. The controller 10 isconfigured to generate a burst dimming signal G2 according to thedimming control signal PWMDIM. With a simplest configuration, thedimming control signal PWMDIM itself may be employed as the burstdimming pulse G2.

The detection resistor R1 is arranged on a path of the LED 6, andspecifically between the cathode of the LED string 6 and the groundterminal.

The oscillator 12 is configured as a variable oscillator. The controller10 is configured to generate a gate pulse signal G1 using a cyclicsignal S1 generated by the oscillator 12. That is to say, the frequencyof the gate pulse signal G1 corresponds to the oscillation frequency ofthe oscillator 12. The cyclic signal S1 may be configured as a pulsesignal, or otherwise a cyclic signal having a sawtooth waveform ortriangle waveform. The kind of the cyclic signal S1 should be designedas appropriate according to the configuration of the controller 10.

The error amplifier EA1 is configured to amplify the difference betweenthe voltage drop V_(R1) that occurs at the detection resistor R1 and areference voltage V_(REF) so as to generate an error voltage V_(FB). Thecontroller 10 is configured to receive the error voltage V_(FB) and thecyclic signal S1, and to generate the gate pulse signal G1 having a dutyratio adjusted such that the voltage drop V_(R1) that occurs at thedetection resistor R1 matches a control voltage V_(DIM). The frequencyof the gate pulse signal G1 corresponds to the frequency of the cyclicsignal S1. A first driver DR1 is configured to drive the switchingtransistor M1 of the DC/DC converter according to the gate pulse signalG1.

A duty ratio detection unit 30 is configured to receive the dimmingcontrol signal PWMDIM, and to detect the duty ratio of the dimmingcontrol signal PWMDIM. Furthermore, the duty ratio detection unit 30 isconfigured to control the oscillation frequency of the oscillator 12according to the duty ratio thus detected. When the duty ratio is large,the duty ratio detection unit 30 is configured to reduce the oscillationfrequency of the oscillator 12, and when the duty ratio becomes small,the duty ratio detection unit 30 is configured to raise the oscillationfrequency of the oscillator 12.

For example, when the duty ratio α thus detected is higher than apredetermined threshold TH, the duty ratio detection unit 30 sets thefrequency of the oscillator 12 to a first value f₁. When the duty ratioα becomes lower than the predetermined threshold TH, the duty ratiodetection unit 30 sets the frequency of the oscillator 12 to a secondvalue f₂ that is higher than the first value f₁. The threshold value THis preferably set to a value that is slightly higher than a lower limitβ shown in FIG. 2.

The above is the configuration of the driving circuit 4. Next,description will be made regarding an example configuration of the dutyratio detection unit 30. FIG. 4A is a circuit diagram which shows afirst example configuration of the duty ratio detection unit 30 shown inFIG. 3. The duty ratio detection unit 30 includes a capacitor C2, ahysteresis comparator 32, a first current source 34, a second currentsource 36, and a switch 38.

The capacitor C2 is arranged such that one end thereof is grounded. Thefirst current source 34 is configured to generate a first current I_(C1)so as to charge the capacitor C2. The second current source 36 isconfigured to generate a second current I_(C2) so as to discharge thecapacitor C2. The switch 38 is a switch configured to disconnect andconnect a path of the second current I_(C2). The one/off state of theswitch 38 is controlled according to the dimming control signal PWMDIM.The second current I_(C2) is represented by the following Expression (1)using a constant K.I _(C2) =K×I _(C1)   (1)

The hysteresis comparator 32 is configured to compare a voltage V₁ thatoccurs at the capacitor C2 with a predetermined threshold voltageV_(TH). The threshold voltage V_(TH) is determined according to theaforementioned threshold value TH. When the output of the hysteresiscomparator 32 is low level, the threshold voltage V_(TH) is set to afirst value V_(TH1). When the output of the hysteresis comparator 32 ishigh level, the threshold voltage V_(TH) is set to a second valueV_(TH2) that is lower than the first value V_(TH1). As an example, thefirst value V_(TH1) and the second value V_(TH2) are represented by thefollowing Expressions.V _(TH1) =V _(DD)×⅔V _(TH2) =V _(DD)×⅓

The output OUT of the comparator 32 represents the comparison resultbetween the duty ratio α of the dimming control signal PWMDIM and thethreshold value. By employing such a hysteresis comparator, such anarrangement is capable of preventing the frequency of the gate pulsesignal G1 from oscillating when the duty ratio α of the dimming controlsignal PWMDIM is in the vicinity of the threshold TH.

FIG. 4B is a waveform diagram which shows the operation of the dutyratio detection unit 30 shown in FIG. 4A. During a period in which thedimming control signal PWMDIM is low level, the switch 38 is turned off,thereby charging the capacitor C2 by means of the first current I_(C1).As a result, the voltage V₁ at the capacitor C2 rises at a sloperepresented by (I_(C1)/C2). With the duty ratio of the dimming controlsignal PWMDIM as α, and with the period thereof as T_(P), the off periodT_(OFF) and the on period T_(ON) are represented by the followingExpressions (2a) and (2b), respectively.T _(OFF) =T _(P)×(1−α)  (2a)T _(ON) =T _(P)×α  (2b)

Thus, the increase in the voltage V₁ at the capacitor C2 for one offperiod T_(OFF) is represented by the following Expression (3).ΔV _(UP) =I _(C1) /C2×T _(P)×(1−α)  (3)

During a period in which the dimming control signal PWMDIM is highlevel, the switch 38 is turned on. In this state, the capacitor C2 isdischarged by means of the current difference between the second currentI_(C2) and the first current I_(C1), i.e., (I_(C2)−I_(C1))=(K−1)·I_(C1).As a result, the voltage V₁ at the capacitor C2 decreases with a sloperepresented by (K−1)·I_(C1)/C2. Thus, the decrease in the voltage V₁ atthe capacitor C2 for one on period T_(ON) is represented by thefollowing Expression (4).ΔV _(DN)=(K−1)·I _(C1) /C2×T _(P)×α  (4)

From Expressions (3) and (4), the amount of change in the voltage V₁ inone cycle is represented by ΔV=ΔV_(UP)−ΔV_(DN)=I_(C1)×T_(P)/C2×(1−K·α).Thus, when K×α=1, ΔV becomes zero. That is to say, the voltage V₁ ismaintained at a constant value at a given level. When α<1/K, the voltageV₁ increases, and when α>1/K, the voltage V₁ decreases. That is to say,the threshold value TH used by the duty ratio detection unit 30 in thecomparison with the duty ratio α of the dimming control signal PWMDIM isrepresented by 1/K. For example, when K=20, judgment is made whether ornot the duty ratio α is higher than 5%.

When the duty ratio α is maintained at a level that is higher than thethreshold value TH=1/K, the voltage V₁ is maintained at zero. When theduty ratio α becomes lower than the threshold value 1/K in a state inwhich the voltage V₁=0 V, the number of cycles N required for thevoltage V₁ to rise from 0 V up to the threshold voltage V_(TH1) isrepresented by the following Expression (5).

$\begin{matrix}\begin{matrix}{N = {{V_{{TH}\; 1}/\Delta}\; V}} \\{= {V_{{TH}\; 1}/\{ {I_{C\; 1} \times {T_{P}/C}\; 2 \times ( {1 - {K \cdot \alpha}} )} \}}} \\{= {{2/3} \cdot {V_{DD}/\{ {I_{C\; 1} \times {T_{P}/C}\; 2 \times ( {1 - {K \cdot \alpha}} )} \}}}}\end{matrix} & (5)\end{matrix}$

For example, each parameter is determined such that, when K=20 and α=0,the required number of cycles N is set to 5.

That is to say, each parameter is determined such that the followingExpression (6) holds true.5=⅔×V _(DD) /{I _(C1) ×T _(P) /C2}V _(DD) /{I _(C1) ×T _(P) /C2}=15/2  (6)

By substituting Expression (6) into Expression (5), the followingExpression (7) is obtained.N=5/(1−20·α)  (7)

Thus, the following relation Expressions are obtained.

When α=1%, N=6.25.

When α=2%, N=8.33.

When α=3%, N=8.33.

When α=4%, N=12.5.

When α=5%, N=25.

As the duty ratio α becomes large, such an arrangement requires anincreased number of cycles N to detect a reduction in the duty ratio α.However, such an arrangement provides a sufficient detection speed forcontrolling the contrast of the liquid display panel.

Conversely, when the duty ratio α is maintained at a level that is lowerthan the threshold value 1/K, the voltage V₁ becomes equal to the powersupply voltage V_(DD). When the duty ratio α becomes higher than thethreshold value 1/K in a state in which the voltage V₁=V_(DD), thenumber of cycles N required for the voltage V₁ to drop from the powersupply voltage V_(DD) down to the threshold voltage V_(TH2)(=V_(DD)/3)is represented by the following Expression (5).

$\begin{matrix}\begin{matrix}{N = {{( {V_{DD} - V_{{TH}\; 2}} )/\Delta}\; V}} \\{= {{2/3} \cdot {V_{DD}/\{ {I_{C\; 1} \times {T_{P}/C}\; 2 \times ( {1 - {K \cdot \alpha}} )} \}}}}\end{matrix} & (8)\end{matrix}$

When α=1, by substituting Expression (6) into Expression (8), theExpression N=200/19 is obtained.

If it is assumed that the frequency of the dimming control signal PWMDIMis configured as a variable frequency having the maximum value I_(PMAX),in such a case, the current I_(C1) and the capacitance C2 may preferablybe determined such that the relation expression I_(C1)/C2=2/15×V_(DD)/T_(PMAX) holds true. Such an arrangement allows the dutyratio to be detected over the possible frequency range. For example,when the dimming control signal PWMDIM is supplied in a range between100 Hz and 500 Hz, the maximum value of the period T_(PMAX) is 10 ms.

FIG. 5 is a circuit diagram which shows a second example configurationof the duty ratio detection unit 30 shown in FIG. 3. By means of digitalsignal processing, a duty ratio detection unit 30 b provides anequivalent operation to that of the duty ratio detection unit 30 a shownin FIG. 4A.

The duty ratio detection unit 30 b includes a flip-flop 40, a counter42, and a digital comparator 44. The flip-flop 40 latches the dimmingcontrol signal PWMDIM at a timing of each positive edge of the clocksignal CLK. It should be noted that the flip-flop 40 may be omitted.When the signal S2 thus latched is high level, the counter 42 decrements(counts down) the count value CNT by (K−1). When the signal S2 thuslatched is low level, the counter 42 increments (counts up) the countvalue CNT by 1.

The counter 42 corresponds to the first current source 34, the secondcurrent source 36, and the capacitor C2 shown in FIG. 4A. The count-downoperation corresponds to the discharging operation shown in FIG. 4A, andthe count-down operation corresponds to the charging operation shown inFIG. 4A. The count value CNT corresponds to the voltage V₁ shown in FIG.4A. The frequency of the clock signal CLK is set to a value that issufficiently higher than the frequency of the dimming control signalPWMDIM. In a case in which the dimming control signal PWMDIM is suppliedin a frequency range between 100 Hz and 500 Hz, the frequency of theclock signal CLK is set to a value on the order of 100 kHz.

The digital comparator 44 corresponds to the hysteresis comparator 32shown in FIG. 4A. When the count value CNT becomes greater than TH₁, thedigital comparator 44 asserts (sets to high level) its output OUT. Whenthe count value CNT becomes smaller than TH₂, the digital comparator 44negates (sets to low level) the output OUT. That is to say, the outputOUT of the digital comparator 44 represents the comparison resultbetween the duty ratio α of the dimming control signal PWMDIM and thethreshold value TH.

For example, in a case in which the digital comparator 44 has an upperlimit MAX of the count value CNT of 1500, the threshold value TH₁ may beset to (⅔×MAX), i.e., 1000, and the threshold value TH₂ may be set to(⅓×MAX). i.e., 500. The upper limit MAX corresponds to V_(DD).

The duty ratio detection unit 30 b shown in FIG. 5 is capable ofcomparing the duty ratio α of the dimming control signal PWMDIM with thethreshold value TH, in the same way as with the duty ratio detectionunit 30 a shown in FIG. 4A.

The above is the configuration of the driving circuit 4. Next,description will be made regarding the operation thereof. Descriptionwill be made returning to FIG. 3. When the light-emitting apparatus 3operates normally, the driving current I_(DRV) is stabilized toI_(DRV)=V_(REF)/R1 (current dimming).

With such an arrangement, the time ratio of a period in which thedriving current I_(DRV) flows through the LED string 6 is controlledaccording to the dimming control signal PWMDIM, thereby adjusting theaverage value of the driving current I_(DRV), i.e., the effectiveluminance of the LED string 6 (burst dimming). With the duty ratio ofthe burst dimming pulse G2 as α, the average value I_(DRV) _(_) _(AVE)of the driving current I_(DRV) is represented by I_(DRV) _(_)_(AVE)=V_(REF)/R1×α.

FIGS. 6A and 6B are waveform diagrams each showing the operation of thelight-emitting apparatus 3 shown in FIG. 3. FIG. 6A shows a case inwhich the duty ratio α indicated by the dimming control signal PWMDIM ishigher than a predetermined threshold value. In this state, thefrequency of the gate pulse signal G1 is set to the first value f₁. Whenthe duty ratio of the dimming control signal PWMDIM is higher than thethreshold value, the frequency of the gate pulse signal G1 is reduced,thereby providing the control IC 100 with reduced power consumption.

FIG. 6B shows a case in which the duty ratio α indicated by the dimmingcontrol signal PWMDIM is lower than a predetermined threshold value. Inthis state, the frequency of the gate pulse signal G1 is set to thesecond value f₂. In this state, such an arrangement ensures that thereare multiple pulses of the gate pulse signal G1 in a short on periodT_(ON) of the burst dimming pulse G2. Thus, such an arrangement iscapable of stabilizing the output voltage Vout at a suitable valueaccording to the duty ratio of the gate pulse signal G1 adjusted bymeans of the feedback operation performed according to the detectionvoltage V_(R1).

FIG. 6C is a diagram which shows the relation between the duty ratio ofthe burst dimming pulse G2 and the driving current I_(DRV) _(_) _(AVE)in the light-emitting apparatus 3 shown in FIG. 3. The solid line (I)indicates the average driving current shown in FIG. 3, and the line ofdashes and dots indicates the average driving current shown in FIG. 1.As described above, by raising the frequency of the gate pulse signal G1in a range in which the duty ratio of the burst dimming pulse G2 is low,such an arrangement is capable of adjusting the average driving currentI_(DRV) _(_) _(AVE) according to the duty ratio indicated by the dimmingcontrol signal PWMDIM even if the duty ratio is in a low range (rangingbetween β′, and β) which is insufficient for the burst dimming operationof conventional arrangements. That is to say, such an arrangement iscapable of driving the LED string 6 at a lower luminance as comparedwith conventional arrangements.

[Second Embodiment]

FIG. 7 is a circuit diagram which shows a configuration of a drivingcircuit 4 a according to a second embodiment. The driving circuit 4 a isconfigured to be capable of driving multiple LED strings 6_1 through6_m, and includes a current driver 8, in addition to the control IC 100a and the output circuit 102.

The current driver 8 includes current sources CS respectively providedto the LED strings 6. Each current source CS includes a transistor M3, adetection resistor R1, and an error amplifier EA2. The transistor M3 andthe detection resistor R1 are arranged in series on a path of thecorresponding LED string 6. The error amplifier EA2 adjusts the voltageat the control terminal of the transistor M3 such that the voltage dropV_(R1) at the first detection resistor R1 matches the control voltageV_(DIM). Each current source CS stabilizes the driving current I_(DRV)that flows through the corresponding LED string 6 as represented byI_(DRV)=V_(DIM)/R1.

The error amplifier EA1 generates a feedback voltage V_(FB) thatcorresponds to the difference between a predetermined reference voltageV_(REF) and the sum total of the voltage drop that occurs at thetransistor M3 and the voltage drop that occurs at the first detectionresistor R1, i.e., the cathode voltage of the LED string 6. The erroramplifier EA1 includes multiple inverting input terminals respectivelyconfigured to receive the cathode voltages of the multiple LED strings6. The error amplifier EA1 amplifies the difference between thereference voltage V_(REF) and the lowest of the multiple cathodevoltages. The controller 10 receives the feedback voltage V_(FB), andgenerates a gate pulse signal G1 having a duty ratio that is adjustedsuch that the lowest cathode voltage matches the reference voltageV_(REF).

The operation of the oscillator 12 and the operation of the duty ratiodetection unit 30 are the same as those of the driving circuit 4 shownin FIG. 3.

The above is the configuration of the driving circuit 4 a. The drivingcircuit 4 a is capable of driving the LED strings 6 at at a lowerluminance compared with conventional arrangements, in the same way aswith the driving circuit 4 shown in FIG. 3.

Description has been made with reference to the embodiments regardingthe present invention. The above-described embodiments have beendescribed for exemplary purposes only, and are by no means intended tobe interpreted restrictively. Rather, various modifications may be madeby making various combinations of the aforementioned components orprocesses. Description will be made below regarding such modifications.

Description has been made regarding an arrangement in which thefrequency of the gate pulse signal G1 is switched between two discretevalues f₁ and f₂. However, the present invention is not restricted tosuch an arrangement. Also, the frequency of the gate pulse signal G1 maybe switched between three or more discrete values. Also, the frequencyof the gate pulse signal G1 may be changed in a continuous range.

Description has been made in the embodiment regarding an arrangement inwhich the dimming control signal PWMDIM is configured as a pulse signal.However, the present invention is not restricted to such an arrangement.For example, the dimming control signal PWMDIM may be configured as ananalog voltage having a voltage level which indicates the duty ratio ofthe burst dimming pulse G2. With such an arrangement, the duty ratiodetection unit 30 can be configured as an analog voltage comparatorconfigured to compare the voltage level of the dimming control signalPWMDIM with the voltages V_(TH1) and V_(TH2) which each correspond tothreshold values.

Description has been made in the embodiments regarding a non-insulatedswitching power supply employing an inductor. Also, the presentinvention can be applied to an insulated switching power supplyemploying a transformer.

Description has been made regarding an electronic device as anapplication of the light-emitting apparatus 3. However, the usage of thepresent invention is not restricted in particular. Also, the presentinvention can be applied to an illumination device and so forth.

The settings of the logical signals, such as the high-level state andthe low-level state of the logical signals, have been described in thepresent embodiments for exemplary purposes only. The settings can befreely modified by inverting the signals using inverters or the like.

Description has been made regarding the present invention with referenceto the embodiment using specific terms. However, the above-describedembodiments show only the mechanisms and applications of the presentinvention for exemplary purposes only, and are by no means intended tobe interpreted restrictively. Rather, various modifications and variouschanges in the layout can be made without departing from the spirit andscope of the present invention defined in appended claims.

The invention claimed is:
 1. A driving circuit configured to supply adriving voltage and a driving current to a light-emitting element, thedriving circuit comprising: a current source arranged on a path of thelight-emitting element; a controller configured to generate a gate pulsesignal having a duty ratio that is adjusted such that voltage dropacross the current source matches a predetermined reference voltage, toreceive a dimming control signal which indicates the duty ratio of theburst dimming, and to generate a burst dimming pulse according to thedimming control signal; a first driver configured to drive, according tothe gate pulse signal, a switching power supply configured to generatethe driving voltage; a second driver configured to switch the stateaccording to the burst dimming pulse between a state in which a path ofthe driving current is connected and a state in which the path of thedriving current is disconnected; and a duty ratio detection unitconfigured to control the frequency of the gate pulse signal accordingto the duty ratio indicated by the dimming control signal, wherein thedimming pulse signal is pulse modulated such that the frequency and theduty ratio thereof respectively indicate the frequency and the dutyratio of the burst dimming operation, and wherein the duty ratiodetection unit comprises: a capacitor arranged such that one end thereofis set to a fixed electric potential; a first current source configuredto charge the capacitor; a second current source configured to beswitchable between an on state and an off state according to the dimmingcontrol signal, and to generate a current that is K times (K representsa real number which is greater than 1) that of the first current sourcein the on state so as to discharge the capacitor; and a comparatorconfigured to compare a voltage that develops at the capacitor with apredetermined threshold voltage.
 2. The driving circuit according toclaim 1, wherein the duty ratio detection unit is configured to raisethe frequency of the gate pulse signal as the duty ratio becomes lower.3. A driving circuit according to claim 1, wherein the comparator isconfigured as a hysteresis comparator.
 4. A light-emitting apparatuscomprising: a light-emitting element; and a driving circuit according toclaim 1, configured to drive the light-emitting element.
 5. Anelectronic device comprising: a liquid crystal panel; and alight-emitting apparatus according to claim 4, arranged as a backlightof the liquid crystal panel.
 6. A driving circuit configured to supply adriving voltage and a driving current to a light-emitting element, thedriving circuit comprising: a current source arranged on a path of thelight-emitting element; a controller configured to generate a gate pulsesignal having a duty ratio that is adjusted such that voltage dropacross the current source matches a predetermined reference voltage, toreceive a dimming control signal which indicates the duty ratio of theburst dimming, and to generate a burst dimming pulse according to thedimming control signal; a first driver configured to drive, according tothe gate pulse signal, a switching power supply configured to generatethe driving voltage; a second driver configured to switch the stateaccording to the burst dimming pulse between a state in which a path ofthe driving current is connected and a state in which the path of thedriving current is disconnected; and a duty ratio detection unitconfigured to control the frequency of the gate pulse signal accordingto the duty ratio indicated by the dimming control signal, wherein thedimming control signal is pulse modulated such that the frequency andthe duty ratio thereof respectively indicate the frequency and the dutyratio of the burst dimming operation, and wherein the duty ratiodetection unit comprises: a counter configured to change its count valueby a first value in a first direction for every edge timing of apredetermined clock signal when the dimming control signal is highlevel, and to change its count value by a second value in a seconddirection for every edge timing of a predetermined clock signal when thedimming control signal is low level; and a digital comparator configuredto compare the count value with a predetermined threshold value.
 7. Thedriving circuit according to claim 6, wherein the digital comparator isconfigured as a hysteresis comparator.
 8. A light-emitting apparatuscomprising: a light-emitting element; and a driving circuit according toclaim 6, configured to drive the light-emitting element.
 9. Anelectronic device comprising: a liquid crystal panel; and alight-emitting apparatus according to claim 8, arranged as a backlightof the liquid crystal panel.